Since 1500 AD, ceramics have been formed by feeding a fluid slip of ceramic materials into a porous mold. The mold absorbs the liquid, thus building up a solid layer or casting which is the "green" (unfired) ceramic part. Slip casting generally takes a long time to form one part and this disadvantage of setting time increases exponentially with increasing part size and which also increases with the use of finer solid materials. Ceramic greenware has also been formed by mixing powders with organic binders, such as wax, which are heated to form a flowable mixture and then fed into a cold mold, thus forming a solid part. With a wax system, it is difficult to heat treat or sinter such greenware without having it melt into a flowable mixture. Also, the parts cannot be used without heat treatment.
Typical industrial practices for making complex-shaped ceramics have used organic liquids for dispersing fine ceramic particulates with polymeric binders and plasticizers for easy forming and handling. However, environmental restrictions and economic concerns shortly required the development of alternative processing methods which do not employ organic solvents or polymeric additives. Polymeric binders and plasticizers also pose problems in injection molding with respect to incomplete binder burnout and excessive burnout time.
Patent prior art of which the applicant is aware includes U.S. Pat. No. 5,190,709 issued to Lukacs, III. This patent describes a one-slip system which contains an organometallic precursor. This precursor needs to be heated to react and form the binder. This process has the disadvantage of using expensive organometallic materials which add so little ceramic to the final product that there is little advantage over using simple organic binders. Furthermore, heat must be transferred to the part to cure the precursor, so forming time is dependent on part size and choice of mold material. U.S. Pat. No. 5,188,780 to Lange et al discloses a method of preparing a dense ceramic product by forming coagulated, weakly attractive ceramic networks. The solid content is increased after coagulation, but before forming. With the Lange process, however, parts must be dried in a non-porous mold for 24 hours or longer. In U.S. Pat. No. 4,624,808 issued to Lange, ceramics are formed by flocculation followed by centrifuging. The actual forming process is centrifuging and the flocculation reaction does not form a part, but is used to increase the effective particle size of the material so it will settle faster. U.S. Pat. No. 4,734,237 issued to Fanelli and Silvers uses an organic gelling agent which sets by cooling. U.S. Pat. No. 2,898,306 to Cramer and Jenkins discloses methods in which alumina gels are formed by slowly increasing the pH of alumina sols. This is achieved through the decomposition of urea to generate ammonia in situ. The reaction requires 10 hours. It is also generally known to form net shape ceramics by flocculation of alumina.
In all of the above-described prior art molding systems, setting times are slow and are dependent upon part size. In many cases, organic or organometallic binding phases are required, necessitating the removal of a carder or removal of heat from the mold to form the part.